devenv: add slow reverse proxy (#16943)

this can be useful when testing timeouts etc
in the dataproxy

ref #16923

devenv/docker/blocks/slow_proxy/Dockerfile

@@ -0,0 +1,7 @@
+
+FROM golang:latest 
+ADD main.go /
+WORKDIR /
+RUN go build -o main . 
+EXPOSE 3011
+ENTRYPOINT ["/main"]